As is well known, a gas turbine engine in its basic form includes a compressor section, a combustion section and a turbine section arranged to provide a generally axially extending flow path for the working gases. Compressed air, from the compressor section, is mixed with fuel and burned in the combustor to add energy to the gases. The hot, pressurized combustion gases are expanded through the turbine section to produce useful work and/or thrust. While an aircraft propulsion engine delivers most of its useful power as thrust, other types of gas turbine engines, typically called auxiliary power units, furnish no thrust but are used to supply compressed air and mechanical power to drive electrical generators or hydraulic pumps. The power produced by any engine is a function of, among other parameters, the temperature of the gases admitted into the turbine section. That is, all other things being equal, an increase in power from a given engine can be obtained by increasing the gas temperature. However, as a practical matter, the maximum gas temperature, and hence the efficiency and output of the engine, is limited by the high temperature capabilities of the various turbine section components.
Within the turbine section are one or more stages of turbine wheel assemblies which are rotated by direct exposure to the hot gases. Such wheels are subjected to very high centrifigual forces and severe thermal gradients as well as high temperatures. There are two basic designs or types of turbine wheels, each having certain operating advantages and disadvantages. An axial-flow wheel has many short, straight blades extending radially from a generally flat disk. Typically the blades are cast individually and attached to a forged disk of different material so that the properties of each component may be optimized for its particular environment. The other basic type of turbine wheel, a radial-inflow wheel, presents more challenging design problems when used in a severe operating environment. Radial-flow wheels are generally one piece with several thin, scrolled blades or fins arranged in a frusto-conical shape somewhat like a common radial centrifugal compressor rotor. In operation, hot combustion gases are directed at the relatively thin blades near the peripheral rim of the wheel and flow inwardly along the hub. The hub surface (and rim) is rapidly heated by the hot gas whereas the interior of the hub responds slowly during a cold engine start. Thus, a transient thermal gardient is created within the hub which causes extreme circumferential compression at the hub surface. When the engine is unloaded or shutdown, the hot gas temperature rapidly drops to a lower level. This reverses the thermal gradient by rapidly cooling the hub surface and rim, thus producing circumferential tension which adds to the tensile stresses produced by centrifugal forces. Such subjection of the rim to high temperature compression and subsequent rapid cooling contraction creates structural cracks, thought to be due to low-cycle thermal fatigue, which leads to eventual destruction of the entire turbine wheel.
Several different approaches to solving or reducing this cracking problem have been tried by prior researchers in this field.
One early attempt involved the addition of cooling air passageways within the turbine wheel adjacent the hot front face. See, for example, U.S. Pat. No. 2,873,945. However, such internal passageways are difficult and/or costly to manufacture.
Another early patent, U.S. Pat. No. 2,919,103, attempts to reduce the temperature of the thin peripheral rim by moving the rim slightly out of the hot gas flow path and directing cooling air towards it.
One of the most successful attempts to reduce the rim cracking problems is shown, but not described, in U.S. Pat. No. 3,163,003. It was discovered that the life of radial-flow turbine wheels could be improved significantly, with only a small loss of aerodynamic efficiency, by removing the thinnest material between the blades or fins near the rim. Such scalloped wheels have now become common in the art. However, the cracking problems were not eliminated and still occur (but after longer times) in the saddle regions between the blades.
More recent attempts to even further extend the life of the scalloped wheels involve the use of two different materials so that the saddle region is crack resistant while the blade tips are resistant to high temperature creep. See, for example, U.S. Pat. Nos. 4,581,300 and 4,659,288. Such dual alloy wheels are difficult and expensive to manufacture. It should be apparent that there is still a need for improvements in radial flow turbine wheels.
Thus it is an object of the present invention to provide a method and apparatus for reducing the range of thermal variation in the saddle region of scalloped radial-flow turbine wheels due to successive cycles of high temperature operation and cool down so that the successive compression and contraction stresses are reduced and thereby extend the life of such wheels.
Another object of the invention is to provide a novel combination of turbine wheel geometry which cooperates with cooling air passages in the surrounding turbine shroud structure so that the thermal fatigue life in the wheel may be increased.
A further object of the invention is to provide a surrounding shroud structure which directs and regulates the flow of cooling air to the more critical portions of the turbine wheel.
Further objects and features of the invention will be apparent from the following specification and claims.